Electric assist steering systems are well known in the art. Electric power assist steering systems that utilize a rack and pinion gear set provide power assist by using an electric motor to either (i) apply rotary force to a steering shaft connected to a pinion gear, or (ii) apply linear force to a steering member having the rack teeth thereon. The electric motor in such systems is typically controlled in response to (i) a driver's applied torque to the vehicle steering wheel, and (ii) sensed vehicle speed.
In U.S. Pat. No. 3,983,953, an electric motor is coupled to the input steering shaft and energized in response to the torque applied to the steering wheel by the vehicle operator. The steering system includes a torque sensor and a vehicle speed sensor. A computer receives the output signals provided by both the torque and speed sensors. The computer controls the amount of steering assist provided by the motor dependent upon both the applied steering torque and the sensed vehicle speed.
U.S. Pat. No. 4,415,054 to Drutchas (now U.S. Pat. No. Re. 32,222,), assigned to TRW Inc., utilizes a D.C. electric assist motor driven through an "H-bridge" arrangement. The assist motor includes a rotatable armature encircling a steering member. The steering member has a first portion with a thread convolution thereon and a second portion with straight cut rack teeth thereon. Rotation of the electric assist motor armature causes linear movement of the steering member through a ball-nut drivably connected to the thread convolution portion of the steering member. A torque sensing device is coupled to the steering column for sensing driver applied torque to the steering wheel. The torque sensing device uses a magnetic Hall-effect sensor that senses relative rotation between the input and output shafts across a torsion bar. An electronic control unit ("ECU") monitors the signal from the torque sensing device and controls the electric assist motor in response thereto. A vehicle speed sensor provides a signal to the ECU indicative of the vehicle speed. The ECU controls current through the electric assist motor in response to both the sensed vehicle speed and the sensed applied steering torque. The ECU decreases steering assist as vehicle speed increases. This is commonly referred to in the art as speed proportional steering.
U.S. Pat. No. 4,660,671 discloses an electric controlled steering system that is based on the Drutchas steering gear. In the arrangement shown in the '671 patent, the D.C. motor is axially spaced from the ball-nut and is operatively connected thereto through a connection tube. The electronic controls includes a plurality of diagnostic features that monitor the operation of the steering system. If an error in the operation of the electric steering system is detected, the power assist system is disabled and steering reverts to an unassisted mode.
U.S. Pat. No. 4,794,997 to North, assigned to TRW Cam Gears Limited, discloses an electric assist steering system having an electric motor operatively connected to the rack through a ball nut. A vehicle speed sensor and an applied steering torque sensor are operatively connected to an ECU. The ECU controls electric current through the motor as a function of both applied steering torque and sensed vehicle speed. The current is controlled by controlling the pulse-width-modulated ("PWM") signal applied to the motor. As the PWM increases, power assist increases. The ECU or computer is preprogrammed with discrete control curves that provide steering assist values (PWM values), also referred to as torque-out values, as a function of applied steering torque, also referred to as torque-in values, for a plurality of predetermined discrete vehicle speed values. Each vehicle speed value as an associated torque-in vs. torque-out control curve.
U.S. Pat. No. 5,257,828 To Miller et al., discloses an electric assist steering system having yaw rate control. This system uses a variable reluctance motor to apply steering assist to the rack member. The torque demand signal is modified as a function of the steering rate feedback.
Known electric assist steering systems have a dynamic performance characteristic, known as the system bandwidth, that varies as a function of vehicle speed. As the vehicle operator applies steering torque and rotates the steering wheel back-and-forth, e.g., left-to-right-to-left, the electric assist motor is energized to provide steering assist commensurate with the steering inputs. How the steering system responds to a particular frequency of back-and-forth steering wheel movement is indicative of the system's dynamic performance.
The amount of local change at the electric assist motor divided by the amount of local change in steering torque applied by the driver is the steering system gain. A time delay occurs from the time steering torque is applied to the steering wheel to the time the assist motor responds. This time delay is a function of the frequency at which the input command is applied. This is referred to as the system response time. The system gain is set to a predetermined value so as to have a short system response time while still maintaining overall system stability. The system response time and system gain determine the system bandwidth.
The bandwidth in known steering systems varies as a function of vehicle speed. If dynamic steering frequency or the "frequency" of a transient response exceeds the system bandwidth at a particular vehicle speed, the steering feel becomes "sluggish" (felt as a "hesitation" when the steering wheel direction is changed) since the steering assist motor can not respond quick enough. Typically, steering system gain as well as system bandwidth decreases as the vehicle speed increases so that system hesitation or sluggishness becomes more noticeable as vehicle speed increases.